1. Field of the Invention
The invention relates generally to a method and apparatus for preventing or minimizing "carry-back". More specifically, it relates to a method and apparatus for preventing carry-back in coal cars or other transportation vehicles.
2. Description of the Prior Art
Carry-back is commonly a cold weather problem and occurs when the ambient temperature is below freezing. An example of carry-back is the cold weather problem of dirt freezing in the bucket of an excavator. The problem occurs, for example, when the temperature of the bucket is below freezing and the bucket is loading material that has been insulated from the ambient temperature (e.g. material which has been below the frost line). When this condition exists, the loaded material may freeze in thin layers to the inside back corners of the bucket. This is sometimes referred to as striatic layering. One of the reasons that striatic layering occurs in the back corners of the bucket is that the ratio of mass between the bucket and the loaded material and surface area contact between that bucket and the material is greatest in the three surface (side, end and floor) back corners of the bucket. With each succeeding dig cycle of the excavator, another thin layer of loaded material will be added (i.e. striatic layering will occur). This further reduces the capacity of the excavator and prevents the excavator from being able to load and dump 100% of its designed capacity. This is obviously undesirable.
The amount of striatic build-up is proportional to the cycle time of the excavator. This means that as the time interval between loading and dumping increases, there is a proportional increase in build-up, so that each freezing layer will incrementally decrease the actual production capacity of the excavator.
The problem of carry-back is well known in various fields, including the field of material transportation. It occurs in coal cars, excavation equipment and other material transportation vessels, vehicles and devices. For simplicity, however, the invention will be described primarily in connection with a coal car, such as depicted at FIGS. 1A-1C. It is to be understood, however, that the invention is not so limited and that one of ordinary skill in the art will be readily able to apply the teachings of this invention to other devices where carry-back is a problem.
Carry-back arises when coal (or other material being transported) adheres to, or otherwise remains in the coal car after a delivery. As a result, the amount of coal which is actually delivered to the destination is less than what is transported. The coal which remains in the coal car after delivery is "carried back" in the coal car when it returns to its point of origin. This is obviously uneconomical. The buyer of the coal does not receive the full amount of coal transported or purchased, the railroad company carries back additional weight and when the coal car returns to its point of origin, the supplier does not have an empty car in which to place a predetermined amount of a particular product.
In coal cars, carry-back may be primarily caused by coal freezing to the coal cars during the colder months (e.g. winter) or due to excess moisture in the coal especially during the warmer or wetter months. At most all times, there is excess surface moisture on the coal.
For example, the typically low-sulfur western coals, in high demand due in part to passage of the Clean Air Act Amendment, are typically 20% to 25% moisture by weight. In addition to the insitu moisture, many points of origin apply additional moisture to the coal through application of water based surfactants to the top of each car load to reduce dust emissions in transit. Other factors may also give rise to the condition of carry-back.
In particular, coal may freeze to the corners of a rail car, starting in the three side corners (side, end and floor). However, depending on the characteristics of the car, carry-back may occur anywhere. For simplicity, the discussion below will refer to the areas where it is desired to prevent or minimize carry-back as the "desired areas".